1. Field of the Invention
The invention relates to the use of sol-gel processes to form silica bodies.
2. Discussion of the Related Art
Optical fiber is produced from a glass preform. As discussed in F. DiMarcello et al. "Fiber Drawing and Strength Properties," Optical Fiber Communications, Vol. 1, Academic Press, Inc., 1995, at 179-248, the disclosure of which is hereby incorporated by reference, the preform is generally arranged vertically in a draw tower such that a portion of the preform is lowered into a furnace region. The portion of the preform placed into the furnace region begins to soften, and the lower end of the preform forms what is known as the neck-down region, where glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fiber. From the lower tip of this neck-down region, the optical fiber is drawn.
Optical transmission fiber typically contains a high-purity silica glass core optionally doped with a refractive index-raising element such as germanium, an inner cladding of high-purity silica glass sometimes doped with a refractive index-lowering element such as fluorine, and an outer cladding of undoped silica glass. In some manufacturing processes, the preforms for making such fiber are fabricated by forming an overcladding tube for the outer cladding, and separately forming a rod containing the core material and inner cladding material. The core/inner cladding are fabricated by any of a variety of vapor deposition methods known to those skilled in the art, including vapor axial deposition (VAD), outside vapor deposition (OVD), and modified chemical vapor deposition (MCVD). MCVD is discussed in U.S. Pat. Nos. 4,217,027; 4,262,035; and 4,909,816, the disclosures of which are hereby incorporated by reference. MCVD involves passing a high-purity gas, e.g., a mixture of gases containing silicon and germanium, through the interior of a silica tube (known as the substrate tube) while heating the outside of the tube with a traversing oxy-hydrogen torch. In the heated area of the tube, a gas phase reaction occurs that deposits particles on the tube wall. This deposit, which forms ahead of the torch, is sintered as the torch passes over it. The process is repeated in successive passes until the requisite quantity of silica and/or germanium-doped silica is deposited. Once deposition is complete, the body is heated to collapse the substrate tube and obtain a consolidated core rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, the overcladding tube is typically placed over the core rod, and the components are heated and collapsed into a solid, consolidated preform, as discussed in U.S. Pat. No. 4,775,401, the disclosure of which is hereby incorporated by reference.
Forming a fiber preform using an MCVD process therefore requires both a substrate tube and an overcladding tube. Previously, both types of tubes were formed from fused quartz or by soot methods, i.e., depositing glass on a mandrel by directing at the mandrel glass particles formed by flame hydrolysis of silicon tetrachloride. Both methods were energy intensive and costly, however, and alternatives were sought.
Because the outer cladding of a fiber is distant from transmitted light, the overcladding glass does not in all cases have to meet the optical performance specifications to which the core and the inner cladding must conform (but still must be substantially free of flaw-inducing refractory oxide particles). For this reason, efforts to both ease and speed manufacture of fiber preforms focused on methods of making overcladding tubes. One area of such efforts is the use of a sol-gel casting process.
U.S. Pat. No. 5,240,488 (the '488 patent), the disclosure of which is hereby incorporated by reference, discloses a sol-gel casting process capable of producing crack-free overcladding preform tubes of a kilogram or larger. In the process of the '488 patent, a colloidal silica dispersion, e.g., fumed silica, is obtained. To maintain adequate stability of the dispersion and prevent agglomeration, the pH is raised to a value of about 11 to about 14 by use of a base. A typical base is tetramethyl ammonium hydroxide (TMAH). Upon introduction of the TMAH, substantially complete dissociation to TMA+ and OH.sup.- occurs, raising the pH value. Other quaternary ammonium hydroxides behave similarly. When the pH is so raised, the silica, it is believed, takes on a negative surface charge due to ionization of silanol groups present on the surface. The negative charge of the silica particles creates mutual repulsion, preventing substantial agglomeration and maintaining the stability of the dispersion. Polymer additives, e.g., binders and lubricants, are included to improve the physical properties of the gelled bodies. As discussed in the Table of the '488 patent, the dispersion is then aged for a time ranging from 1 to 20 hours.
Subsequent to aging, as discussed in Col. 15, lines 39-65 of the '488 patent, a gelling agent such as methyl formate is added to the dispersion to lower the pH. The methyl formate reacts with the water and/or base to generate H.sup.+ ions that neutralize the negative character of the silica particles. The hydrolysis of the ester occurs over approximately 10 minutes, at which time enough ions are formed to neutralize the silica to a degree where gellation is induced. (Gellation, as used herein, indicates that the colloidal silica particles have formed a three-dimensional network with some interstitial liquid. Existence of such a three-dimensional network is typically indicated when the dispersion becomes essentially non-flowing, e.g., exhibiting solid-like behavior, at room temperature.) Typically, once the gelling agent is added, but before gellation occurs, the mixture is pumped into a tubular mold containing a central mandrel, and the gel is aged in the mold for 1 to 24 hours. The mandrel is removed, and the gelled body is then extracted from the mold, typically by launching the body from the mold in water to prevent breakage. The body is then dried, fired to remove volatile organic materials and water, and then sintered to form the finished overcladding.
While useful overcladding bodies are obtained by processes such as that of the '488 patent, the casting process is relatively slow and requires simultaneous casting in hundreds of molds to produce the amount of tubes required for commercial feasibility. In addition, in some circumstances, it is desired to reduce the amount of polymer additives typically added to casting formulations. Processes that require lesser amounts of such additives would therefore be advantageous.
Sol-gel casting methods such as that of the '488 patent typically have not been used to produce commercial substrate tubes. Specifically, substrate tubes have thin walls, e.g., about 5 mm thick prior to drying and sintering, and therefore require stronger gel bodies than overcladding tube. Yet, the stronger tubes needed are extremely difficult to cast as a gel without encountering slumping or breaking of the tubes during post-extrusion processing.
Improved sol-gel methods, useful for fabricating both overcladding tubes and substrate tubes, are therefore desired.